Metal Vacancy Ordering in an Antiperovskite Resulting in Two Modifications of Fe2SeO

Small, red Fe2SeO single crystals in two modifications were obtained from a CsCl flux. The metastable alpha-phase is pseudo-tetragonal (Cmce, a = 16.4492(8) angstrom, b = 11.1392(4) angstrom, c = 11.1392(4) angstrom), whereas the beta-phase is trigonal (P3(1), a = 9.8349(4) angstrom, c = 6.9591(4) angstrom)) and thermodynamically stable within a narrow temperature range. Both crystal structures were solved from twinned specimens. The enantiomers of the beta-phase appear as racemic mixtures. Selenium and oxygen form two individual interpenetrating primitive cubic lattices, giving a bcc packing. A quasi-octahedrally coordinated iron atom is found close to the center of each surface of the selenium sublattice. The difference between the alpha- and beta-phases is the distribution of iron at 2/3 of the surfaces. alpha- and beta-Fe2SeO are comparable with metal-vacancy-ordered anti-perovskites. Each Fe/O lattice can also be described in terms of vertex-sharing OFe4 tetrahedra, with a crystal structure similar to that of an antisilicate. Iron is divalent and has a high-spin d(6) (S = 2) configuration. The beta-phase exhibits magnetoelectric coupling.